It is known to manufacture hollow metallic articles by superplastic forming and diffusion bonding workpieces. These workpieces include elementary metal, metal alloys, intermetallic materials and metal matrix composites. At least one of the workpieces must be capable of superplastic extensions.
In one known process the surfaces of the workpieces to be joined are cleaned, and at least one surface of one or more of the workpieces is coated in preselected areas with a material to prevent diffusion bonding. The workpieces are arranged in a stack and the edges of the workpieces are welded together, except where a pipe is welded to the workpieces, to form an assembly. The pipe enables a vacuum, or inert gas pressure, to be applied to the interior of the assembly. The assembly is placed in an autoclave and heated so as to "bake out" the binder from the material to prevent diffusion bonding. The assembly is then evacuated, using the pipe, and the pipe is sealed. The sealed assembly is placed in a pressure vessel and is heated and pressed to diffusion bond the workpieces together to form an integral structure. Diffusion bonding occurs when two mating surfaces are pressed together under temperature, time and pressure conditions that allow atom interchange across the interface so that the interface effectively ceases to exist. The first pipe is removed and a second pipe is fitted to the diffusion bonded assembly at the position where the first pipe was located. The integral structure is located between appropriately shaped dies and is placed within an autoclave. The integral structure and dies are heated and pressurised fluid is supplied through the second pipe into the interior of the integral structure to cause at least one of the workpieces to be superplastically formed to produce an article matching the shape of the dies.
The superplastic forming and diffusion bonding process may be used to produce contoured articles for example fan blades, or fan duct outlet guide vanes, for gas turbine engines by twisting an integral structure formed by the diffusion bonding process before superplastically forming the integral structure. A procedure for twisting the integral structure before superplastic forming is disclosed in our prior UK patent application No. 9209464.8 filed on 1 May 1992, the contents of which are incorporated herein by reference.
A problem with the procedure disclosed in our earlier patent application, mentioned above, is that the contoured article produced by the superplastic forming and diffusion bonding process requires further operations to produce the finished article. In particular there are superfluous portions of material on the contoured article at regions which correspond to the leading edge and trailing edge of the finished fan blade, or outlet guide vane. This excess material is removed from the contoured article by rough machining to within about 1 mm of the final required dimensions of the leading edge and trailing edge regions of the finished fan blade, or outlet guide vane, and then by belt grinding to the final required shape and dimensions.
This process of removing excess material from the contoured article to produce the finished article is very labour intensive and inefficient. Furthermore there is a risk of removing too much material from the contoured article with the result that the finished article is rejected due to none conformance with dimensional constraints. If this occurs the expensive diffusion bonding and superplastic forming processes have been wasted.